Simulation study on in-situ pyrolysis behavior of tar-rich coal under overburden stress

SHI Qingmin; YANG Xiaolong; WANG Shuangming; KOU Bingyang; WANG Shengquan; SHI Yaya; YANG Fu; LI Jiuqing

doi:10.13225/j.cnki.jccs.2025.0280

Abstract：The oil and gas conversion of tar-rich coal in-situ pyrolysis provides a new idea to improve the domestic oil and gas independent guarantee and push the green and low-carbon industry of coal.The behavior of tar-rich coal pyrolysis un-der crustal stress is distinct from conventional ground pyrolysis,leading to significant differences in oil and gas output law,but there are few relevant studies.Tar-rich coal pyrolysis simulation experiments under different overburden stresses were conducted,and the influence of the crustal stress on pyrolysis deformation,pore structure evolution,and molecular structure disparities was analyzed using low-temperature N2 adsorption,X-ray diffraction,and high-resolution transmis-sion electron microscopy.The effect mechanism of the different stress loadings on the pyrolysis of tar-rich coals was ex-plored.The results showed the tar-rich coal in-situ pyrolysis properties exhibited two stages as the stress loading increased.During the low-stress loading stage(0～10 MPa),the lack of effective radial confining pressure on the coal samples resul-ted in the enhancement of axial stress constantly fracturing the coal,improving pore connectivity and pyrolysis fluid re-lease ability.On the one hand,it was conducive to the formation of large pores during convection,and the number of＞50μm pyrolysis macropores increased significantly;on the other hand,it was conducive to the reduction of the chance of the secondary reaction of pyrolysis fluid,which led to the improvement of tar yield and the growth of the coal molecular struc-ture,which was reflected in the gradual decrease in interlayer spacing,the gradual increase in stacking height and lateral size,and the increase in random vitrinite reflectance from the optical properties.However,the compaction of coal and the closure of fissures at the high-stress loading stage(＞10 MPa),inhibit internal pyrolysis volatile migration;on the one hand,it is easier to form a relatively small pore size of 2-25 μm under the weak convection;on the other hand,it strengthens the degree of secondary reaction of pyrolysis fluids,leading to a reduction in the tar yield and an increase in the gas and semi-coke yields.Furthermore,X-ray diffraction and high-resolution transmission electron microscopy data in-dicated that the swelling deformation of the coal matrix caused by the continuous high-pressure stagnant flow was not fa-vorable to the orderly growth of pyrolysis semi-coke aromatic structure.